In a photosensitive member cartridge on which a developing cartridge is mounted, as disclosed in JP-A-2000-250310, a plurality of rotatable rollers are disposed in a developing cartridge accommodating portion. When the developing cartridge is attached in the developing cartridge accommodating portion, the weight of the developing cartridge is received on an opposite side of a roller shaft of a developing roller to a photosensitive drum by the rollers. Further, with the rollers, rattling is suppressed which otherwise occurs when attaching and detaching the developing cartridge.